I think the similarity (in my mind) is the "you can't be charged for overages" aspect of it..
Use as much as you can, you won't be charged any more than X dollars per month..
But past a certain point, you will be throttled.. Not cut off, but slowed..
And back in the ISP days, we would actively disconnect high-usage users to free up phone lines during peak times (which was typically 6-10pm on weekdays and 1pm to 10pm on weekends)
Depending on how old you are, you may or may not remember a time when ISPs advertised their "user-to-line ratios", meaning we didn't have a phone line for every customer - we maintained a 7:1 ratio which was considered good back then).
So in order to allow the lower-usage customers to get online to check their emails or surf after dinner, we had to make sure that there were lines available.
If you had already used more than (I think - memory is fuzzy here) 150 hours per month, then you were subject to being disconnected during peak hours.
So in effect, we were also throttling them, which is similar to what wireless carriers do today I think...